The Anonymous online hacking group claims to have defaced almost 500 websites in China to protest against internet restrictions there.
Targets included government sites, official agencies, trade groups and many others.
A message put on the sites urged Chinese people to join Anonymous and stage their own protests against the regime.
There has been no official confirmation of the defacements. News agencies reported that government officials had denied any had taken place.
China tries to block internet users from seeing social media sites such as Facebook and Twitter.
Information on politically sensitive topics is also often blocked.